This disclosure relates to a method and system for embedding information onto a substrate. More particularly, this disclosure is related towards a method and system for infrared watermarking using modified Gray Component Replacement/Under Color Removal (GCR/UCR) schemes and thus will be described with particular reference thereto. However, it should be appreciated that some embodiments are amenable to other applications.
By way of background, watermarking is a common way to enable security and other features in document production. This technique permits the insertion of information in the form of digital image signals into documents. This information may include copyright notices, security codes, identification data, bar codes, etc. This information may be hidden in images and exposed through various methods. This data may also be grouped in bits describing the information pertaining to a signal which can be read by a signal reader. Most common watermarking methods for images work in spatial or frequency domains.
It is desirable for this data to remain hidden under normal visible light for practical and aesthetic purposes. It is also desirable to provide an infrared reading method that is capable of exposing the hidden data once it employs rendering techniques. The traditional approach is to render the encoded data with special inks that are not visible under normal light but have strong distinguishing characteristics under certain types of spectral illumination. However, these special inks and materials are often difficult to incorporate into standard electro-photographic or other non-impact printing systems.
Generally, the same visual color can be achieved with different amounts and combinations of respective available colorants. This can easily be understood when considering that the human visual system—to a good approximation—can be modeled by a three component system, whereas printing is commonly achieved in a four or more component printing system. This underdetermined situation offers additional degrees of freedom that can be otherwise utilized. A common terminology used in the context of representing a three component color system with a four or more component rendering system is “metameric rendering”. Therefore, when reading in an input color, various amounts of different toner may be used in order to match the desired effect. In the infrared scenario, a generalization can be performed to arbitrary input images since the infrared characteristic is dominated by carbon black toner presence. This is the case even when the toner uses a continuous feed despite the flash fusing requiring a high absorption for melting. Consequently, many early examples of infrared watermarking use two different black strategies and switched between the two is a function of the watermark. In order to indicate the effectiveness of the infrared absorption, images in the prior art suffered from strong artifacts in image reproduction. The differences in color were visible in the two different areas, making the effect unusable. The main source of artifact seems to be the different color range and different black strategies as well as the gamut mapping induced difference in output color.
There have been attempts in the art to overcome the aforementioned difficulty. One technique includes creating an infrared mark employing different infrared transmission characteristics of four or more different printing colorants. This creates an infrared mark by printing a first colorant combination with high infrared reflectance in close proximity to a second colorant with the same visual response under visible light while having a different infrared reflectance. This method, however, does contain some drawbacks inherent within the process. One drawback includes a limited color palette because it is difficult to produce many colors under visible light that have the desired response under infrared light and thus the general inability to use this approach for infrared encoding of natural scene and other images. Also, often when these colors are placed in close proximity, artifacts can still be seen despite their relatively similar appearance under visible light. Another drawback to this method includes when attempting to correct the artifacts, oftentimes there is a low strength of the watermark in many areas. This in part due to the intent of color matching using two different color gamuts, one with high infrared absorption ability and the other with low infrared absorption ability.
Other attempts have been made in order to correct apparent fault in the prior art, one of which is to create watermarks that closely align with the image being created. In this form the watermark can be hidden in shadows that are cast by the image itself. This, however, limits the watermarks effectiveness as arbitrary marks cannot be obtained. The watermarks in this embodiment must generally be related to the image which is attempting to be portrayed.
Therefore, there is a need in the industry for an infrared watermarking solution that can be performed to arbitrary input images. There is also a need in the industry for a solution to have a minimal impact on system overhead requirements, including storage and digital processing requirements. Furthermore, it is desirable that the solution be obtained without physical modification to the printing devices and without the need for costly special upgrades in materials and media formats. This disclosure solves the above-referenced difficulties as well as many others.